FIG. 7 is a circuit diagram of a conventional reference oscillator. In reference oscillator 100 in FIG. 7, oscillator 101 is, for example, an AT-cut quartz crystal. Driver circuit 102 connected in parallel to oscillator 101 is configured, for example, with a CMOS inverter. Load capacitances 103 and 104 are connected to oscillator 101 and ground.
In general, the reference oscillator used for a reference frequency of communications devices, such as mobile phones, or high-frequency receiving devices, such as TV sets, require frequency stability against ambient conditions. In particular, frequency stability against temperature changes is one important performance. For example, TV sets require stability of at least ±60 ppm or less is required in a use temperature range. A structure of reference oscillator 100 is effective for achieving this performance, and crystal oscillator 101 is an essential device for equipment requiring highly accurate frequency stability.
However, crystal oscillator 101 has a structure of suspending an oscillating portion in midair while holding a part of crystal piece cut to a predetermined shape. Accordingly, downsizing is difficult. In addition, a device having the above structure needs to be manufactured one by one. This makes cost reduction difficult.
To redeem the disadvantage of reference oscillator 100 made of crystal, an oscillator using a silicon oscillator utilizing a semiconductor manufacturing process has been disclosed. A reference oscillator employing a silicon oscillator is configured in the same way as that in FIG. 7. However, since a temperature coefficient of silicon material is large, an oscillation frequency varies in line with a temperature change. Therefore, a temperature sensor is used for detecting a change in ambient temperature so as to apply temperature compensation control for retaining a constant frequency.
FIG. 8 is a block diagram of a conventional oscillator unit. In FIG. 8, conventional oscillator unit 201 includes reference oscillator 202 for generating a reference oscillation signal, synthesizer 204 for outputting a local oscillation signal based on the reference oscillation signal output from this reference oscillator 202, temperature sensor 205 for detecting temperature, and controller 206. Controller 206 adjusts a frequency of the local oscillation signal output from synthesizer 204 based on a detection result of temperature sensor 205. This controller 206 applies temperature compensation control for adjusting output frequency of synthesizer 204 based on a temperature detection result of reference oscillator 202 detected by temperature sensor 205. This prior art is disclosed, for example, in Patent Literature 1.
A temperature coefficient of silicon oscillator (not illustrated) in reference oscillator 202 is 30 ppm/° C., which is large. Therefore, a frequency adjustment level output from controller 206, corresponding to the detection result of temperature sensor 205, becomes large. As a result, a frequency variation in the local oscillation signal output from synthesizer 204 becomes large.
On the other hand, in a high-frequency receiving device, a frequency of high-frequency signal received is converted to an intermediate frequency signal, using the local oscillation signal obtained by converting a signal output from the oscillator. This is demodulated in a later process. Stable frequency without variation is thus demanded for this intermediate frequency signal. Accordingly, in case of using the oscillator in the high-frequency receiving device, a demodulator may not be able to demodulate if frequency greatly varies in the intermediate frequency signal as a result of temperature compensation control. Therefore, the oscillator unit using an oscillator with large temperature coefficient cannot be used in a field of high-frequency receiving devices, such as mobile phones and broadcast receiving tuners, even if a temperature compensation control circuit is added.
Patent Literature 1: U.S. Pat. No. 7,145,402